Frequency modulation



July 11, 1933.

W, VAN B. ROBERTS FREQUENCY MODULATION Filed July 10, 1930 MHHMM INVENTOR WALTER VAN B. ROBERTS ATTORNEY Patented July 11, 1933 UNETED arsnr caries WALTER VAN B. BGBERTS, F PRINCETON, NEW JERSEY, ASS-IGNOR TO RADIO CURPORATTON 0F DEYLAWAR-E FREQUENCY IvZQDULATION Application filed July 10,

vision of new and useful methods and meansfor producing frequency modulated energy.

If an electron discharge device has, in its anode circuit, a resistance, it will be found that the input circuit thereof oifers a capaci-v tive reactance, variable with variations in the resistance. More broadly, the combination of the device and resistance may be considered a network, and int r-electrode capacity of the tube furnishing capacity and, the external resistance of the tube, resistance.

Variation of resistance of thenetwork will, in accordance with what has been said, vary the effective capacityof the network.

It is a further object of this invention to make use of the foregoing principles in a frequency modulation system and this is done, according to my invention, by coupling anetwork including a resistance and a capacity to. the frequency control circuit of a source of oscillations. By varying the resistance of the network, the effective capacity thereof is varied which in turn varies the resonant frequency of the control circuit and hence the frequency of oscillations generated.

By making the capacity of the network the inter-el ctrode capacity of an electron discharge device, and by varying the external resistance of the device, frequency modulated high frequency energy may readily be obtained. In one form'of my invention the external resistance may simply be a microphone which is varied in value according to modulation energy, but preferably, I make the external resistance in the form of an auxiliary electron discharge device whose internal impedance or resistance is varied according to modulating potentials applied to a control electrode thereof.

My invention is more fully described in connection with the accompanyin drawing which is given only by way of illustration and is not to be considered in any way limitative. In the drawing,

Figure 1 diagrammatically indicates an electron discharge device having in its anode circuit a variable resistance and is given in 1930. Serial No. 466,955.

order to explain th-eunderlying principles of my invention, I

Figure 2 illustrates one form of my invention wherein a microphone is used as an anode resistance, and,

Figure 3 illustrates a preferred form of my invention.

In Figure 1, I have illustrated an electron discharge device or vacuum tube 2, having a cathode or filament 4, control electrode or grid 6 and anode or plate 8. The interelectrode capacity of the tube has been indicated diagrammatically by the dotted line capacity 10. In the output circuit or inthe anode circuit of the device 2 there is placed a variable resistance 12. With the arrangement as shown, the input impedance across terminals 14: will be capacitive in nature. It will be found that by variation of resistance 12, the fictitious capacity across terminals 1e will vary.

Now, as shown in Figure 2, by connecting the input terminals 14 across the frequency control ortunable circuit 16 of an oscillator 18, and by replacing resistance 12 with a,

microphone 20, voice or modulating energy striking the microphone will cause the fictitious capacity paralleling the frequency control circuit 16 to vary. This in turn will alter the tuning of the frequency control circuit 16 and consequently the frequency of the high frequency energy generated by the oscillator 18.

The output of the oscillator 18 may, of course, be fed to a power amplifier 24 and then supplied to any desired utilization or output circuit 26.

In a preferred form of my invention, as.

illustrated in Figure 3, the external resistance of electron discharge device 2 whose in put terminals, and consequently whose fictitious input capacity reactance, is paralleled with a tunable frequency control circuit 16 for regeneratively connected oscillator 28, takes the form of an auxiliary electron discharge device or vacuum tube 30. The internal impedance or resistance of the device is varied as shown by applying modulation potentials to the control electrode 32 thereof, either voice energy by means of a microoil:

phone 34 or keyed alternating energy supplied by the action of a keying device 36 and local oscillator 38.

As in Figure 2 the frequency of the control circuit 16 is varied by variation of the resistance of the network formed by the ca pacity of tube 2, augmented if desired by condenser 39, together with the resistance of tube 30. The resulting frequency modulated energy generated by oscillator 28 is amplified with asuitable power amplifier 40 and may be radiated by a suit-able radiating antenna 41.

It is to be understood that the invention as described is not limited to the form shown. The invention may be applied to any fre quency modulating system and even to one wherein vacuum tubes are not involved at all, in which case simple capacitors and resistors will be used, variation in a resistance producing variation in the effective capacity of the network.

In "putting the invention into use, the following formulae are useful:

Apparent input capacity= 1 C l- 0 ,(1 um) approximately.

Whence R =external plate circuit resistance of tube.

By means of these formulae, a, C R and R may be so chosen that a given change (5R produced by the modulating voltage will produce a desired change (Z0 and hence a desired amount of change in frequency.

Having thus described my invention, What I claim is:

A thermionic oscillator comprising, an electron discharge tube and a frequency determining network, said oscillator generating a frequency depending in part upon the capacity between two points in said network, a second thermionic tube having an anode, cathode, an anode cathode circuit including resistance, a control electrode and a capacity between said anode and said control electrode, whereby the apparent capacity between the control grid and cathode of said tube depends upon the resistance in circuit between the anode and cathode of said tube, means for connecting the grid and cathode of said tube to said points in said network of said oscillator, a circuit for varying the resistance in the anode circuit of said second named tube comprising, a thermionic tube having its cathode connected to the anode of said second named tube and its anode connected to the cathode of said second named tube, and means for varying the anode to cathode impedance of said last named tube comprising, a source of modulating potential having one terminal connected to the grid electrode of said last named tube and the other terminal connected to the cathode of said last named tube, whereby the frequency of the oscillations produced by said oscillator is varied in accordance with said modulating potential.

WALTER VAN B. ROBERTS. 

